In a known exhaust gas purification apparatus for the internal combustion engine using a NOX occlusion and reduction catalyst, the NOX component in the exhaust gas is occluded (the word “occlusion” is used herein as a concept including both “absorption” and “adsorption”) when the air-fuel ratio of the exhaust gas flowing into the catalyst is lean, and the occluded NOX is reduced and purified using the reduction component in the exhaust gas when the air-fuel ratio of the exhaust gas flowing into the catalyst becomes a stoichiometric or rich air-fuel ratio.
In the presence of SOX (sulfur oxide) in the exhaust gas, the NOX occlusion and reduction catalyst is known to occlude SOX in exactly the same manner as NOX in the case where the air-fuel ratio of the exhaust gas is lean.
The SOX has a high affinity with the occluded NOX component and generates a very stable compound. Once SOX is occluded in the NOX occlusion and reduction catalyst, therefore, the occluded SOX is not substantially released from the NOX occlusion and reduction catalyst simply by setting the air-fuel ratio of the exhaust gas to a rich air-fuel ratio and, thus, SOX is accumulated gradually in the catalyst.
In other words, in the normal process for occlusion and purification by reduction of NOX, the SOX occluded in the NOX occlusion and reduction catalyst is substantially not released. With an increase in the amount of SOX occluded, therefore, the NOX occlusion capacity (the maximum NOX amount that can be occluded by the NOX occlusion and reduction catalyst) of the NOX occlusion and reduction catalyst decreases in accordance with the SOX amount occluded. With the increase in the amount of SOX occluded by the NOX occlusion and reduction catalyst, therefore, the NOX occlusion and reduction catalyst can no longer sufficiently occlude the NOX contained in the exhaust gas, and a so-called sulfur poisoning phenomenon (SOX poisoning) occurs in which the NOX purification rate is remarkably reduced.
In order to prevent the SOX poisoning of the NOX occlusion and reduction catalyst, a poisoning restoration process is required in which the occluded SOX is released from the NOX occlusion and reduction catalyst and the amount of the occluded SOX is reduced.
As described above, however, SOX occluded in the NOX occlusion and reduction catalyst forms a compound far more stable than NOX and cannot, substantially, be released from the NOX occlusion and reduction catalyst and, therefore, SOX cannot be sufficiently released from the NOX occlusion and reduction catalyst simply by setting the air-fuel ratio of the exhaust gas flowing into the NOX occlusion and reduction catalyst to a rich air-fuel ratio.
Normally, therefore, the poisoning restoration process is executed in such a manner that the exhaust gas is set to a rich air-fuel ratio while at the same time maintaining the NOX occlusion and reduction catalyst in a high-temperature range.
As described above, however, SOX is difficult to release from the NOX occlusion and reduction catalyst, and cannot be released sufficiently from the NOX occlusion and reduction catalyst within a short time even by the poisoning restoration process for holding the NOX occlusion and reduction catalyst in a high-temperature environment with a rich air-fuel ratio. A protracted execution of the poisoning restoration process, on the other hand, poses the problem that the increased length of time during which the catalyst is exposed to the high temperature is liable to cause thermal deterioration of the catalyst.
On the other hand, it is known that, in the case where hydrogen is contained in the exhaust gas flowing into the NOX occlusion and reduction catalyst during the poisoning restoration process, the rate at which SOX is released from the NOX occlusion and reduction catalyst increases and the poisoning restoration process can be efficiently completed within a comparatively short time.
Normally, the method of supplying hydrogen to the exhaust gas, in addition to the method of supplying hydrogen stored in an external container to the exhaust gas, includes a method in which hydrogen is generated from HC, CO or H2O contained in the exhaust gas by the water-gas-shift reaction.
In the case where the air-fuel ratio of the exhaust gas of the engine becomes rich, for example, the water-gas-shift reaction (CO+H2O→CO2+H2) or steam reforming (HC+H2O→CO2+H2) occurs and hydrogen is generated from HC, CO or H2O generated at the time of combustion. These reactions are further promoted by a three-way catalyst or the like. In the ordinary internal combustion engine having a three-way catalyst as a start catalyst in the exhaust path upstream of the NOX occlusion and reduction catalyst, for example, a comparatively large amount of hydrogen is generated in the exhaust gas once the exhaust gas air-fuel ratio becomes rich at the time of the poisoning restoration process or the reduction of NOX occluded in the NOX occlusion and reduction catalyst.
Apart from the three-way catalyst, it is possible to generate hydrogen at the time of engine operation with a rich air-fuel ratio by a hydrogen generating catalyst arranged in the exhaust path to cause the water-gas-shift reaction, or steam reforming, efficiently.
In Japanese Unexamined Patent Publication (Kokai) No. 2002-47919 ('919 publication), for example, although the SOX poisoning restoration process is not described, the degeneration of the NOX occlusion and reduction catalyst is determined using the hydrogen component generated in the exhaust gas at the time of purification by reduction of the NOX occluded in the NOX occlusion and reduction catalyst.
At the time of purification by reduction of the NOX occluded in the NOX occlusion and reduction catalyst, hydrogen in the exhaust gas is consumed for reducing NOX and therefore no hydrogen flows out downstream of the NOX occlusion and reduction catalyst as long as NOX remains occluded in the catalyst.
When hydrogen begins to flow out into the exhaust gas downstream of the NOX occlusion and reduction catalyst, therefore, the whole amount of the NOX occluded in the NOX occlusion and reduction catalyst is considered to have been purified by reduction. Therefore, the time from the start of purification by reduction of the occluded NOX to the detection of the hydrogen component on the downstream side corresponds to the amount of NOX occluded by the NOX occlusion and reduction catalyst.
According to the '919 publication, a H2 sensor for detecting hydrogen in the exhaust gas is arranged in the exhaust paths upstream and downstream of the NOX occlusion and reduction catalyst and, based on the time required from the detection of hydrogen by the upstream-side H2 sensor to the detection of hydrogen by the downstream-side H2 sensor during the process of purification by reduction of the occluded NOX, it is determined whether the amount of NOX occluded by the NOX occlusion and reduction catalyst has decreased or not (whether the NOX occlusion and reduction catalyst is degenerated or not).
As described above, the poisoning restoration process can be efficiently executed by supplying hydrogen into the exhaust gas flowing into the NOX occlusion and reduction catalyst during the execution of the SOX poisoning restoration process.
For this purpose, however, a sufficient amount of hydrogen to release the whole amount of SOX occluded in the NOX occlusion and reduction catalyst from the catalyst is required to be supplied to the NOX occlusion and reduction catalyst during the execution of the poisoning restoration process. Also, in order to supply a sufficient amount of hydrogen to the NOX occlusion and reduction catalyst, a lengthy SOX poisoning restoration process is required in the case where the concentration of the hydrogen component in the exhaust gas is low.
Further, in the case where the concentration of the hydrogen component in the exhaust gas supplied to the NOX occlusion and reduction catalyst exceeds a predetermined value during the poisoning restoration process, the problem is posed that the extraneous SOX reacts with hydrogen and H2S (hydrogen sulfide) is generated. Hydrogen sulfide not only has a unique odor but also is toxic, and therefore it is not desirable that hydrogen sulfide is generated each time the poisoning restoration process is executed.
For this reason, the concentration of the hydrogen component in the exhaust gas supplied to the NOX occlusion reproduction catalyst during the SOX poisoning restoration process and the duration of the poisoning restoration process are required to be set in such a manner that a sufficient amount of hydrogen can be supplied to the NOX to release and reduce the SOX occluded by the NOX occlusion and reduction catalyst on the one hand, and the concentration of the hydrogen component in the exhaust gas is required to be lower than the value for generating hydrogen sulfide on the other hand. In other words, the concentration of the hydrogen component in the exhaust gas during the SOX poisoning restoration process and the duration of the restoration process are required to be controlled in an appropriate range taking the aforementioned facts into consideration.
However, none of the prior arts considers the requirement of controlling the concentration of the hydrogen component in the exhaust gas during the SOX poisoning restoration process and the duration of the restoration process as related to the concentration of the hydrogen component. Although the '919 publication refers to the detection of the hydrogen component in the exhaust gas using the H2 sensor, it totally fails to consider the efficient execution of the restoration process using hydrogen during the SOX poisoning restoration process or the operation of detecting the concentration of the hydrogen component and controlling the concentration of the hydrogen component itself or the duration of the restoration process.
As a result, in the prior art, there is a problem that the SOX poisoning restoration process of the NOX occlusion and reduction catalyst cannot be efficiently executed.